The successful recovery of trace or contact DNA is highly variable. It is seemingly dependent on a wide range of factors, from the characteristics of the donor, substrate and environment, to the delay between contact and recovery. There is limited research on the extent of the effect these factors have on trace DNA analysis. This study investigated the persistence of trace DNA on surfaces relevant to the investigation of burglary and robbery offences. The study aimed to limit the number of variables involved to solely determine the effect of time on DNA recovery. Given that it is difficult to control the quantity of DNA deposited during a hand contact, human buffy coat and DNA control solution were chosen as an alternative to give a more accurate measure of quantity. Set volumes of these solutions were deposited onto outdoor surfaces (window frames and vinyl material to mimic burglary and ‘bag snatch' offences) and sterile glass slides stored in a closed environment in the laboratory, for use as a control. Trace DNA casework data was also scrutinised to assess the effect of time on DNA recovery from real samples. The amount of DNA recovered from buffy coat on the outdoor surfaces declined by approximately half over two weeks, to a negligible amount after six weeks. Profiles could not be obtained after two weeks. The samples stored in the laboratory were more robust, and full profiles were obtained after six weeks, the longest time period tested in these experiments. It is possible that profiles may be obtained from older samples when kept in similarly favourable conditions. The experimental results demonstrate that the ability to recover DNA from human cells on outdoor surfaces decreases significantly over two weeks. Conversely, no clear trends were identified in the casework data, indicating that many other factors are involved affecting the recovery of trace DNA. Nevertheless, to ensure that valuable trace evidence is not lost, it is recommended that crime scenes are processed expeditiously.

Trace evidence characteristics of DNA: A preliminary investigation of the persistence of DNA at crime scenes

When forensic scientists evaluate and report on the probative strength of single DNA traces, they commonly rely on only one number, expressing the rarity of the DNA profile in the population of interest. This is so because the focus is on propositions regarding the source of the recovered trace material, such as ‘the person of interest is the source of the crime stain’. In particular, when the alternative proposition is ‘an unknown person is the source of the crime stain’, one is directed to think about the rarity of the profile. However, in the era of DNA profiling technology capable of producing results from small quantities of trace material (i.e., non-visible staining) that is subject to easy and ubiquitous modes of transfer, the issue of source is becoming less central, to the point that it is often not contested. There is now a shift from the question ‘whose DNA is this?’ to the question ‘how did it get there?’. As a consequence, recipients of expert information are now very much in need of assistance with the evaluation of the meaning and probative strength of DNA profiling results when the competing propositions of interest refer to different activities. This need is widely demonstrated in day-to-day forensic practice and is also voiced in specialized literature. Yet many forensic scientists remain reluctant to assess their results given propositions that relate to different activities. Some scientists consider evaluations beyond the issue of source as being overly speculative, because of the lack of relevant data and knowledge regarding phenomena and mechanisms of transfer, persistence and background of DNA. Similarly, encouragements to deal with these activity issues, expressed in a recently released European guideline on evaluative reporting [42], which highlights the need for rethinking current practice, are sometimes viewed skeptically or are not considered feasible. In this discussion paper, we select and discuss recurrent skeptical views brought to our attention, as well as some of the alternative solutions that have been suggested.

https://serval.unil.ch/resource/serval:BIB_97C76CB9C546.P001/REF.pdf

Evaluation of Forensic DNA Traces When Propositions of Interest Relate to Activities : Analysis and Discussion of Recurrent Concerns

Summary

When examining a sample of glass fragments recovered from a suspect in a forensic case, many questions arise: “Did this man break that window?”, “Are these fragments from the crime scene source?”, “Do the fragments recovered from the suspect come from more than one source?”, “How common is it to find glass on someone unrelated with crime?” etc. Such questions are usually answered with the help of statistical methods. This paper reviews some of the statistical solutions and problems encountered in the interpretation and evaluation of forensic glass evidence.

Resume

En examinant un echantillon de fragments de verre provenant d'un suspect dans une affaire legale, de nombreuses questions se posent: “Cet homme a-t'il brise la fenetre?”, “Ces fragments proviennent-ils du lieu du crime?”“Les fragments retrouves sur le suspect ont-ils plus d'une origine?”“Est-il courant de trouver du verre sur quelqu'un sans rapport avec un crime”, etc. On repond habituellement de telles questions avec l'aide de methodes statistiques. Cet article passe en revue des solutions statistiques et des problemes rencontres dans l'interpretation et l'evaluation de la preuve legale du verre.

https://www.jstor.org/stable/info/1403825

S.A. Coulson

Papers

Glass on clothing and shoes of members of the general population and people suspected of breaking crimes